1. Field of the Invention
This invention relates to a micrometer of a digital display type. More particularly, the invention relates to a micrometer having a spindle that is displaced in the axial direction during rotation that intends cost reduction, operability improvement, and accuracy improvement.
2. Description of the Related Art
As compared with a measuring instrument of a mechanical graduation display type, a measuring instrument of an electronic digital display type has become pervasive in general measuring instruments in view of the advantage of the superior precision and ease of reading.
Digital display in which a rotary encoder is built is increasingly adopted for a micrometer of a type wherein a spindle displaces in the axial direction while it rotates with rotation of a thimble.
A related digital display type micrometer has a structure including a complete mechanical graduation display mechanism so that it can be used even if the lifetime of a power source (battery) for driving an encoder for detecting the displacement amount of a spindle as an electric signal, electric circuitry, a digital display, etc., comes to an end. That is, the micrometer includes a structure wherein an outer sleeve and a thimble are provided on the frame side and the spindle side respectively, a main scale graduation is formed along the axial direction of the outer circumference of the outer sleeve, and an vernier scale graduation is formed along the outer circumference of the thimble. As a result, the related micrometer provides the disadvantages of complicated assembly and high costs.
In the related structure, there is also the disadvantage for measurement in that, for example, if the operator holds a measured object by left hand, grasps the frame by right hand, and displaces the spindle away from an anvil while turning a ratchet knob with the thumb and forefinger of the right hand, the ratchet knob moves away from the frame gradually and thus the micrometer becomes difficult to handle when the spindle is displaced largely.
Then, the applicant previously proposed a micrometer overcoming the above disadvantages (refer to U.S. Pat. No. 5,829,155).
This micrometer includes a U-shaped frame having an anvil at one end, a spindle screwed into an opposite end of the frame and displaced in the axial direction with the screwed rotation thereof, an encoder for detecting the axial displacement amount of the spindle from the rotation amount of the spindle, and a digital display for digitally displaying the measurement value based on an output signal of the encoder. The micrometer further includes a thimble provided at the opposite end of the frame and being rotatable around an axis of the spindle at an axially constant position of the spindle. Further, a ratchet mechanism is placed between the thimble and the spindle for transmitting the rotation of the thimble to the spindle and allowing the axial displacement of the spindle, and when a predetermined or more load is put on the spindle, causing the thimble to be raced with respect to the spindle.
According to the micrometer thus structured, the main scale graduation, vernier scale graduation, and the like of the related micrometer are omitted, so that the costs required for working the graduations, etc., can be reduced. Moreover, the outer sleeve for forming the main scale graduation can also be made unnecessary, thus the number of parts and the number of assembly steps can be decreased. Therefore, the costs can be reduced.
In the measurement, if the thimble is rotated, the rotation of the thimble is transmitted via the ratchet mechanism to the spindle, so that the spindle is displaced in the axial direction. At this time, even if the spindle is displaced away from the anvil, the thimble is rotatably placed at the constant position of the frame, the thimble is not displaced with the spindle. Accordingly, even if the spindle is displaced largely, a disadvantage of degrading the operability does not arise.
However, in the structure of the related digital display type micrometer, the ratchet mechanism is constructed by a ratchet ring fixed in the thimble and having a serrated protrusion on the inner peripheral surface and a plate spring inserted between the ratchet ring and the outer peripheral surface of the spindle (the outer peripheral surface formed with an external thread). Additionally, the plate spring is retained in a V groove formed along the axial direction of the outer peripheral surface of the spindle at one end and is pressed and urged by the serrated protrusion of the ratchet ring at an opposite end. Namely, the spindle has a structure that the V groove is formed along the axial direction of the outer peripheral surface formed with the external thread.
In such a structure, when the spindle rotates, the V groove formed on the outer peripheral surface of the spindle acts as a tap for thread working an internal thread on the frame side, which threadably engages with the external thread of the spindle. Thus, as the micrometer is used, a clearance is produced between the external thread and the internal thread, causing the degradation of the precision of the micrometer.